A fuel cell is an electrochemical cell which generates electricity by combination of hydrogen and oxygen. Unlike a general chemical cell, the fuel cell can generate electricity continuously as long as the necessary hydrogen and oxygen are supplied. In addition, the fuel cell has no heat loss so that efficiency of the fuel cell is twice as high as efficiency of internal combustion engine. Furthermore, since the fuel cell directly converts chemical energy generated by combination of hydrogen and oxygen into electric energy, the fuel cell is eco-friendly, and is capable of being operated without worries about the exhaustion of fossil fuel.
Based on the type of electrolyte, the fuel cell may be classified into polymer electrolyte fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, and alkaline fuel cell. These fuel cells are basically operated based on the same principle. However, the kind of fuel, operation temperature, catalyst, and electrolyte for each kind of fuel cell may be different. Among the aforementioned fuel cells, the polymer electrolyte fuel cell may be operated at a relatively-lower temperature, and may be formed in a compact size owing to a large output density, whereby the polymer electrolyte fuel cell is suitable for a transport system as well as a small-sized mounting type generating apparatus.
One of the most important methods to improve efficiency of the polymer electrolyte fuel cell is to maintain moisture content by supplying predetermined moisture to polymer electrolyte membrane of membrane-electrode assembly. This is because the generating efficiency is rapidly lowered together with dry of the polymer electrolyte membrane.
A method for humidifying the polymer electrolyte membrane is a membrane humidifying method for supplying moisture to flowing gas through the use of polymer separation membrane.
The membrane humidifying method uses a membrane which selectively permeates only vapor contained in unreacted gas, to thereby supply the vapor contained in the unreacted gas to the polymer electrolyte membrane. This method is advantageous in that it can manufacture a small-sized humidifier with lightness in weight.
If the selective permeation membrane used for the membrane humidifying method forms a module, it is preferable to use hollow fiber membranes having a large permeation area per unit volume. That is, if fabricating the humidifier with the hollow fiber membranes, the hollow fiber membranes having a large contact surface area can be highly integrated so that the fuel cell is sufficiently humidified even with small volume. In this case, the humidifier with the hollow fiber membranes can be fabricated of a low-priced material. Also, moisture and heat contained in unreacted gas discharged at a high temperature from the fuel cell may be collected and reused in the humidifier.
FIGS. 1 and 6 illustrate a humidifier 100 and 300 for fuel cell 100 according to the related art. FIG. 1 illustrates the tube-shaped humidifier 100, and FIG. 6 illustrates the square pillar-shaped humidifier 300. As shown in FIGS. 1 and 6, the humidifier 100 and 300 for fuel cell includes a membrane housing 110 and 310 with a bundle of hollow fiber membranes 120 and 320. In this case, a first inlet 141 and 351 for reaction gas to be supplied to the fuel cell (not shown) is formed at one side of the membrane housing 110 and 310; and a second outlet 152 and 352 for supplying humidified reaction gas to the fuel cell is formed at the other side of the membrane housing 110 and 310. Also, a second cap 150 and 350 having a second inlet 151 and 351 for moisture-containing unreacted gas discharged from the fuel cell is provided at one side of the membrane housing 110 and 310; and a first cap 140 and 340 having a first outlet 142 and 342 for discharging the unreacted gas supplied to the inside of the membrane housing 110 and 310 through the second inlet 151 and 351 is provided at the other side of the membrane housing 110 and 310. Meanwhile, plural second holes 112 and 312 are formed on the outer circumferential surface of the membrane housing 110 and 310 at which the second cap 150 and 350 having the second inlet 151 and 351 is installed.
The aforementioned humidifier 100 and 300 for fuel cell is disadvantageous in that the moisture-containing unreacted gas supplied through the second inlet 151 and 351 is not uniformly distributed to the entire bundle of the hollow fiber membranes 120 and 320. That is, since most of the supplied moisture-containing unreacted gas maintains straightness, the flow of the supplied moisture-containing unreacted gas is concentrated on the holes near to the second inlet 151 and 351 among the plural second holes 112 and 312. As a result, the moisture is supplied only to the reaction gas flowing in the hollow of the hollow fiber membranes being in contact with the moisture-containing unreacted gas, and is not supplied to the reaction gas flowing in the hollows of the remaining hollow fiber membranes. Accordingly, the remaining hollow fiber membranes are rarely utilized for the humidifying process, to thereby deteriorate the humidifying efficiency of the humidifier 100 and 300.
Especially, as shown in FIG. 7, unlike the tube-shaped humidifier 100, the square pillar-shaped humidifier 300 has no obstacles therein, whereby most of the unreacted gas supplied through the second inlet 351 maintains straightness. Thus, the unreacted gas is concentrated to the hollow fiber membranes near to the second inlet 351, whereby the humidifying efficiency of the square pillar-shaped humidifier 300 is more lowered.